The present invention relates to dispersive optical systems employing transmissive diffractive gratings, and particularly to optical systems with increased dispersion achieved by passing the light twice through the transmissive diffraction grating.
Dispersive systems that utilize diffractive gratings (DG) are used in several fields, including spectroscopic and communication systems. One of the most important characteristics of such systems is their linear dispersion, i.e., the linear separation between different wavelengths in the system""s image plane. The linear dispersion plays the key role in defining the resolution of the spectral apparatus, or the capacity of the communication system.
The linear dispersion is proportional to the angular dispersion of the DG and the focal length of the optics that transform the angular dispersion into the linear dispersion. An increase in linear dispersion of a system by a proportional increase in the focal length of the optics will unavoidably lead to a proportional increase in the size and cost of the optical elements and the system as a whole. An alternative approach to increase the linear dispersion of the system is to increase the angular dispersion of the dispersion component used in the system. The angular dispersion of a grating is inversely proportional to the spacing between the grating""s grooves, so that reducing the spacing leads to an increase in the angular dispersion of the grating and in the linear dispersion of the entire system. Unfortunately, a reduction of the spacing between the grating grooves has limitations based on fabrication tolerances and manufacturability.
Thus it would be desirable to provide a dispersive optical system with an increased linear dispersion without increasing the focal length of the optics or decreasing the spacing of the grating structure.
From a manufacturing perspective, it is advantageous to fabricate a grating on a flat surface rather than a curved one. Transmissive diffractive gratings on flat substrates can be made economically by standard lithographic techniques.
It is an object of the present invention to provide a dispersive system based on a transmissive diffractive grating with increased linear dispersion without increasing the grating line frequency and preferably without increasing the system""s dimensions and without sacrificing the image quality of the output spectrum.
The increase in linear dispersion is achieved by passing the light twice through the grating. The system is designed such that on each pass through the grating the dispersion of the system is increased. Preferably, the linear dispersion of the system is increased approximately twice compared to that of a system with a single transmissive diffraction grating with the same line frequency. Further, this increase in the linear dispersion of the system is preferably achieved without increasing the dimensions of the optical components or the size of the entire system.
Simply passing the light twice through a transmissive DG does not assure an overall increase in a system""s dispersion. Rather, the optics of the system must be chosen so that the dispersive effects of the two passages are additive. In addition, it is also necessary to meet the system specifications for image quality of the output spectrum.
In order to meet system specifications and to achieve enhanced dispersion, optical systems employing the present invention are optimized using standard techniques commonly employed in the design of imaging systems. The element spacings, their dimensions, material properties, groove geometry, and the working order of diffraction can be used as variables in the optimization procedure to meet performance targets based on the device specification and manufacturability requirements. In particular, the shape of the grooves of the transmissive grating (i.e., the groove""s blaze angle, width, and height) is selected to maximize the efficiency of the light diffracted in the working order and to minimize the light getting into diffraction orders other than the working order using standard techniques known in the art. Thus, the hi and wi values in FIG. 1 and FIG. 2 are selected on the basis of desired performance criteria and desired diffraction efficiency.
The systems of the invention can be used to either disperse composite light into its components (e.g., in a wavelength demultiplexer) or to combine light of different wavelengths into a composite (e.g., in a wavelength multiplexer).
When used to disperse light, the invention provides an optical system for dispersing composite electromagnetic radiation into dispersed electromagnetic radiation comprising:
(a) an input for the composite electromagnetic radiation and an output for the dispersed electromagnetic radiation;
(b) a transmissive diffractive grating;
(c) optics to provide conjugate imagery between the input and the output; and
(d) a reflector to pass the radiation twice through the diffraction grating.
When used to combine light, the invention provides an optical system for combining dispersed electromagnetic radiation into composite electromagnetic radiation comprising:
(a) an input for the dispersed electromagnetic radiation and an output for the composite electromagnetic radiation;
(b) a transmissive diffractive grating;
(c) optics to provide conjugate imagery between the input and the output; and
(d) a reflector to pass the radiation twice through the diffraction grating.
In accordance with certain embodiments of the invention, the dispersing/combining aspects of the invention are used together to produce an add-drop system. In accordance with these embodiments, the invention provides an add-drop device which comprises:
(a) an input for input signals at a plurality of wavelengths;
(b) an output for output signals;
(c) a transmissive diffractive grating;
(d) optics to provide conjugate imagery between the input and the output; and
(e) a reflector which: (i) reflects at least one selected wavelength of the plurality of wavelengths so that said wavelength passes through the diffractive grating twice; and (ii) transmits the remainder of the plurality of wavelengths so that those wavelengths do not pass through the diffractive grating twice.
In a preferred embodiment, the invention provides an add-drop system comprising:
(a) a system input for input signals at a plurality of wavelengths;
(b) add inputs for add signals at a plurality of add/drop wavelengths;
(c) a system output for output signals;
(d) drop outputs for drop signals at the plurality of add/drop wavelengths;
(e) a first transmissive diffractive grating;
(f) a second transmissive diffractive grating;
(g) a reflector which: (i) reflects the plurality of add/drop wavelengths; and (ii) transmits the remainder of the plurality of wavelengths other than the plurality of add/drop wavelengths; and
(h) optics which: (i) passes the drop signals from the system input through the first transmissive diffractive grating twice to disperse the plurality of add/drop wavelengths to the drop outputs; (ii) passes the add signals from the add inputs through the second transmissive grating twice so that the plurality of add/drop wavelengths from the add inputs have substantially no dispersion at the system output; and (iii) passes the remainder of the plurality of wavelengths from the system input through the first and second transmissive gratings so that those wavelengths have substantially no dispersion at the system output.